In high-frequency electronic circuits, there is often a need to change the frequency of the transmit signal, i.e., to shift from one frequency to another, in order to prevent interference to and from other RF transceiver units or other sources of noise. Such frequency shifting may be achieved by varying the bias applied to a varactor diode, that in turn, changes the capacitance values disposed in a crystal oscillator circuit.
At the receiving end, a synchronous sampler, such as used in a microwave impulse radar (MIR), is used to downconvert the received RF signal to an audio frequency signal by a scale factor. For example, to convert a transmitted RF signal having a frequency of 6.5 GHz, to an audio signal having a frequency of 5.000 KHz, a scaling factor of 1,300,000 is used. Transmitter 10 of FIG. 1 transmits an RF signal TX_RF, which is received by receiver 20 having disposed therein synchronous sampler 12. All synchronous signals that are received by synchronous sampler 12 are converted and combined into a composite audio signal, RX_audio, by the same scaling factor.
FIG. 2 shows a frequency spectrum of received signals 25 and 30 having respective center frequencies of 6.50 GHz and 6.510 GHz. Signal 30 represents a noise signal that is also received by receiver 20. Assuming a scaling factor of 1,300,000, the downconverted audio signal corresponding to noise signal 30 has a frequency of 5.008 KHz. FIG. 3 shows the frequency spectrum of downconverted audio signals 35 and 40, respectively corresponding to RF signals 25 and 30. To attenuate audio noise signal 40, while allowing received RF signal 35 to pass at the receiving end, a bandpass having a frequency characteristics shown with dashed line 45 is used.